Docker Volume Mounting: How to find path in container?

Question

I have a python script that I have placed inside a docker container named "grapher". The python script inside the "grapher" container generates a graph and saves it like so:

# CODE ABOVE THIS MAKES PLOT #
plt.draw()
filename = "digraph" + str(self.count) + ".png"
plt.savefig(filename)

I want to access these saved figures on my computer, so I am attempting to use "volumes" inside my docker-compose file. The problem is, all the tutorials I find say I need to include the "path in the container". And then the tutorials just magically know what file path to use.

How the heck do I figure out what filepath my container is using?? I've made a bunch of file location guesses based off the tutorials I've found, one of which caused Ubuntu 18.04 to black-screen-of-death (whoops...). I am totally lost. I've included a snippet of my docker-compose.yml file below.

version: '3.0'
services:
  # OTHER CONTAINERS ABOVE THIS#
  grapher:
     build: ./Grapher
     depends_on:
       - hmi_pass_thru
     volumes:
      - graph-data:/home/vic/Documents/5ExtraExtraNodes/Grapher
     network_mode: host

volumes:
  graph-data:
networks:
  test_net:
    external: true

Please help.

Edit 1: My main confusion is that I don't have a file system inside my container. My container is just running the python script. So how do I tell what my "path in container" is?

Edit 2 : My DockerFile that the "grapher" container is built from:

FROM python:3

WORKDIR /home/vic/Documents/5ExtraExtraNodes/Grapher

COPY requirements.txt ./
RUN pip install --no-cache-dir -r requirements.txt

COPY . .

CMD ["python","-u","pcap_grapher3.py"]

Edit 3 : Results of exec

docker exec -it 5fe4dc4 /bin/bash
root@vic-Capstone:/home/vic/Documents/5ExtraExtraNodes/Grapher# ls -a
.          digraph14.png  digraph21.png  digraph29.png  digraph36.png  digraph43.png  digraph8.png
..         digraph15.png  digraph22.png  digraph3.png   digraph37.png  digraph44.png  digraph9.png
Dockerfile     digraph16.png  digraph23.png  digraph30.png  digraph38.png  digraph45.png  pcap_grapher3.py
digraph1.png   digraph17.png  digraph24.png  digraph31.png  digraph39.png  digraph46.png  requirements.txt
digraph10.png  digraph18.png  digraph25.png  digraph32.png  digraph4.png   digraph47.png
digraph11.png  digraph19.png  digraph26.png  digraph33.png  digraph40.png  digraph5.png
digraph12.png  digraph2.png   digraph27.png  digraph34.png  digraph41.png  digraph6.png
digraph13.png  digraph20.png  digraph28.png  digraph35.png  digraph42.png  digraph7.png
root@vic-Capstone:/home/vic/Documents/5ExtraExtraNodes/Grapher# 

Answer 1

I believe you are getting the syntax for volumes backwards. If you are looking to save files from your container onto you host, bind the volumes as host:container .

You can define whichever path you want inside your container's file system. Then, you save to that location.

Since you copy your files over with

COPY . .

Your script is copied to the container directory relative to the WORKDIR . Mounting this directory will allow you view/edit/save files to your host machine.

From the docs:

volumes:
  # Just specify a path and let the Engine create a volume
  - /var/lib/mysql

  # Specify an absolute path mapping
  - /opt/data:/var/lib/mysql

  # Path on the host, relative to the Compose file
  - ./cache:/tmp/cache

  # User-relative path
  - ~/configs:/etc/configs/:ro

  # Named volume
  - datavolume:/var/lib/mysql

An example Dockerfile which may help explain it better than me:

FROM python:3.8-slim-buster

# all code will be inside this directory in the container
WORKDIR /app

COPY requirements.txt requirements.txt
RUN pip3 install -r requirements.txt

# copy all code in present working directory on HOST to /app/. on the CONTAINER
COPY . .

CMD [ "python3", "-m" , "flask", "run", "--host=0.0.0.0"]

References:

https://docs.docker.com/compose/compose-file/compose-file-v3/#volumes

https://docs.docker.com/language/python/build-images/